Spong, in a copending application entitled "Information Record and Related Recording and Playback Apparatus and Methods", Ser. No. 688,495, filed Mar. 19, 1976 and incorporated herein by reference, describes an ablative recording system whereby a focussed modulated light beam, such as a laser beam, is directed at an ablative recording medium. The recording medium comprises a light reflecting material coated with a light absorbing material on a substrate. The thickness of the light absorbing layer is chosen to reduce the reflectivity to a minimum value so that a maximum of light energy impinging on it is retained therein and is converted to thermal energy. This thermal energy causes the light absorbing material in the area struck by the light to ablate, thereby exposing selected portions of the light reflecting layer. During readout, the contrast between the light reflected from the absorbing layer, which is at the reflection minimum, and the light reflecting layer is detected.
Ongoing work in this area has resulted in the improved performance of the materials employed. Thus, in an illustrative embodiment of this recording medium, a substrate which is a flat, smooth, non-conductor of heat is coated with a thin layer of a light reflecting material, such as aluminum. The aluminum layer is passivated as described in a copending application entitled "Ablative Optical Recording Medium" by Bartolini et al, Ser. No. 668,504, filed Mar. 19, 1976. The passivated aluminum layer is in turn coated with a layer of an organic light absorbing material, such as 4-phenylazo-1-naphthylamine, as described in Bloom et al, "Ablative Optical Recording Medium", U.S. Pat. No. 4,023,185.
Alternatively, the light reflecting layer is coated with a transparent dielectric material, such as silicon dioxide. A thin layer of a metal is coated thereon to serve as the light absorbing layer. This configuration is described in the copending application of Bell entitled, "Information Record", Ser. No. 782,032, filed Mar. 28, 1977. Titanium is the metal most frequently used for this embodiment.
In order to eliminate or reduce signal defects or dropouts caused by surface dust which precipitates onto the medium from the environment, an overcoat from about 0.05 to 1 millimeter thick is applied to the light absorbing layer as described in a copending application entitled "Thick Protective Overcoat Layer For Optical Video Disc" by Bloom et al, Ser. No. 828,815 filed concurrently herewith and incorporated herein by reference. Dust particles and other surface contaminants which settle on the upper surface of the overcoat layer are so far removed from the focal plane of the recording lens that their effect on the recording or playback signal is considerably reduced, and no defects are noticeable on the playback monitor.
Silicone resin is a good overcoat material. However, the preferred silicone resin system uses a platinum catalyst to promote curing of the resin. Platinum can react with amines present in the light absorbing layer, thereby adversely affecting curing of the resin and attacking the surface of the light absorbing layer. This reaction, which increases the number of signal defects or dropouts, can be mitigated but not eliminated by baking or ageing the dye-coated disc before applying the silicone overcoat. In addition, organic dyes dissolve in most organic solvents, thereby limiting the number of materials suitable for use as overcoats.
When a metal light absorbing layer is used, it must have a low melting point to avoid damage to or optical distortion of the overcoat layer during recording. Thus, high-melting metals which form otherwise excellent light absorbing layers cannot be used effectively for recording through the overcoat layer. An improved recording medium would make it possible to protect the overcoat layer from thermal or chemical interaction with the light absorbing layer.